routing: add BlindedPayment to unifiedEdge

Later on in this series, we will need to know during path finding if an
edge we are traversing was derived from a blinded payment path. In
preparation for that, we add a BlindedPayment member to the
`unifiedEdge` struct.

The reason we will need this later on is because: In the case where we
receive multiple blinded paths from the receipient, we will first swap
out the final hop node of each path with a single unified target node so
that path finding can work as normal. Once we have selected a route
though, we will want to know which path an edge belongs to so that we
can swap the correct destination node back in.

routing/pathfind.go

@@ -968,6 +968,7 @@ func findPath(g *graphParams, r *RestrictParams, cfg *PathFindingConfig,
 				inboundFee,
 				fakeHopHintCapacity,
 				reverseEdge.edge.IntermediatePayloadSize,
+				reverseEdge.edge.BlindedPayment(),
 			)
 		}